FIG. 1 illustrates, in partial schematic diagram form and partial block diagram form a class A driver circuit 10 in accordance with the prior art. Class A driver circuit 10 includes P-channel transistors 14 and 18, current source 16, differential amplifier 12, current source 24, N-channel transistor 26, Miller compensation capacitor 22, and zero nulling resistor 20. Prior art class A driver circuit 10 is for driving a load impedance 28.
Class A driver circuit 10 includes two stages. A first stage includes differential amplifier 12 and a second stage includes current source 24 and N-channel transistor 26. The Miller compensation circuit includes series-connected resistor 20 and capacitor 22, and connects an output terminal of differential amplifier 12 to an input terminal of the second stage amplifier. Miller compensation is typically used to provide stability to the output terminal of differential amplifier 12. P-channel transistors 14, 18, and current source 16 provide a current source for differential amplifier 12.
Prior art class A driver 10 may be used in a variety of applications. A class A driver of this type is used in high performance systems requiring good drive strength with high linearity. The class A amplifier is also useful for driving loads that are highly resistive in addition to being capacitive. It may also be important for a class A amplifier to provide relatively low power consumption, such as in battery powered applications.
Typically, the circuit design for this type of class A driver is optimized for a worst case load. Depending upon the particular application, load impedance 28 may have a resistive component between, for example, 400 ohms (.OMEGA.) and 100 kilo-ohms (K.OMEGA.). In this case, the 400 .OMEGA. load may be considered the worst case load. However, if the application requires a 100 K.OMEGA. load, the current requirements to drive load impedance 28 would be reduced so that the second stage driver, which was designed for the worst case load of 400 .OMEGA., would waste a lot of power driving the 100 K.OMEGA. load.